In the office machines as printers, copy machines and other similar to them as well as in the printing presses, the record carriers in a sheet form are most often kept in stock in a pile. Then every uppermost sheet of the pile is selected and fed into the office machine. The selection, or the separation of the uppermost sheet, from the next sheet in the pile is a technically difficult problem, especially in the case of sheets of paper because they stick to each other in the pile. The clinging of the sheets is affected by adhesion, electrostatic discharge, friction, and other similar agents and depends on a great number of factors as, for instance, the thickness of the sheet, the stiffness of the sheet, the texture of the surface of the sheet, the humidity of the air, etc.
It is known from DE 44 44 836 A1 and DE 100 16 793 A1, that a rolling action should be applied to the uppermost sheet of the pile in order to loosen the uppermost sheet and to move it in the direction of the feeding. In the process of this rolling, rolling elements, for example, turning rollers are moved over the uppermost sheet exerting pressure over the pile in the direction of the movement of the sheet. In this way the uppermost sheets of the pile are deformed by this rolling action, whereas the deformation of the uppermost sheet is greatest. By means of this deformation, air can penetrate between the uppermost sheet and the following second sheet in such a way that the uppermost sheet can be loosened from the second sheet. By means of the pressure that the rolling elements exercise over the pile, a buckle in the upper sheets is formed in front of the rolling elements, which runs before the rolling element, whereby the rolling element exerts a feeding force onto the uppermost sheet in the direction of the feeding. In this way, the upper sheets of the pile are fanned out in the form of scales, or, respectively shingles, in the direction of the feeding.
The separation of the sheets using a rolling element has in particular the advantage that the separation takes place to a considerable extent independently of the properties of the sheets such as the thickness of the sheet, the stiffness of the sheet, the texture of the surface of the sheet, etc. The separation functions in a very reliable way, so that the undesirable pick-up of the second sheet of the pile can be excluded to a large extent. Since the separation does not depend very much on the quality of the sheets, this method of separation is suitable above all for such office machines, in which different sheet formats and sheet quality are stocked and selectively fed into the machine.
In the equipment that is known, the upper sheets of the pile are fanned out in the form of shingles by the effect of the rolling. As soon as the front edge of the uppermost sheet has been pushed at a sufficient distance from the front edge of the next, the second sheet, the uppermost sheet can be seized at its front edge and can be transported further. The fanning out of the upper sheets by means of the rolling action requires different times depending on the quality of the sheets.
Further, it is known that sheets from a pile can be selected in a way that a separation roller seizes by means of traction the uppermost sheet in the pile and then pushes it against a slanted ramp. Such devices are known, for instance, from EP 0534 245 A1, DE-PS 493 270, WO 89/03 798, and U.S. Pat. No. 6,227,534. In these devices, the front edge of the uppermost sheet is being lifted onto the slanted ramp by the separation roller by means of traction, whereby the front edge bends upwards and is separated from the second sheet that can eventually be selected. Since in this device, the uppermost sheet is pushed from the pile by means of traction, the uppermost sheet does not get loosened from the next one, the second sheet in the pile, so that the probability that the undesirable pick-up of the second sheet takes place is greater. The separation of the uppermost sheet from the second one is done essentially by bending the front edge upwards onto the slanted ramp, which hence should not rise too steeply. This upward bending is essentially depending on the angle of the upward gradient of the ramp and the properties of the sheet, and particularly on the stiffness of the sheet. That is why these devices are suitable only for a relatively narrow range of sheet qualities, or, respectively, need to be correspondingly adjusted to the different sheet qualities.